This way we can instruct update_3rdparty to download a specific version
of pdfjs, e.g. to make debugging easier or to match the version of a
system package.
Syntax:
update_3rdparty.py -p 1.2.109
or
update_3rdparty.py --pdfjs=1.2.109
If the command line argument is not given, the script will automatically
download the latest release.
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